9.
pupil. The diameter of this is the size of
the object lens, divided by the magnification.
Thus the exit pupil of our 8 x 30's is just
over 3mm, and the larger this is, the better
it is for it increases the light value. This
exit pupil is extremely important when
ordering a pair of binoculars. For maximum
efficiency it should be at least as large as
the pupil of your own eye under the worst
conditions in which the glasses are liable
to be used.
People's pupils vary considerably,
normally averaging 7mm in early childhood,
about 6mm in adolescence, 5mm in young adults,
4mm in middle age, 3mm in later middle life
and 2mm in old age. You see, under bright
conditions the pupil gets smaller, in dull
light it gets bigger and the younger the
person the greater will be the difference
between the two measurements. For example,
the average pupil size in a young adult at
night is about 8mm and hence it is the reason
why a 7 x 50 glass has been chosen by the
Navy, where a considerable amount of obser-
vation is required at night. The light value
of a 7 x 50 glass is several times greater
than the 8 x 30 we spoke of earlier, for its
exit pupil (50 / 7) is 7mm and can be used
at night. My own glasses are 7 x 50's. A
binocular with an exit pupil smaller than
the user's eye-pupil can be very irritating.
The hands are rarely quite steady with the
result that the object is being constantly
cut off in the field of view, due to the
relative motions between the binoculars and
the eyes. The higher the magnification and
the greater the weight of the binoculars,
the more apparent this will be. The bigger